trade fees api errors

[matejsp]

There are some inconsistencies based on specification: https://docs.kucoin.com/#actual-fee-rate-of-the-trading-pair

GET /api/v1/trade-fees?symbols=BTC-USDT,KCS-USDT

Issues:

1. symbols is said to be optional (it is not -> 400 error)
2. symbols can be listed up to 10 (if more than 1, comma separated -> 401 error)
3. no way to get a list of symbols with a different that base-fee